As the structures of wind power plants increase in size, it is often necessary to implement electric drives of the wind power plants in a modular manner. This is the case, for instance, in connection with wind power plants, in which a wind turbine is allowed to rotate at a variable speed depending on wind conditions. A generator run by the wind turbine produces electric energy that is supplied to the grid. As the speed of the wind turbine varies, the frequency of the electric energy produced by the generator also varies. Hence, at the generator output there has to be connected equipment wherewith the frequency and amplitude of the produced electricity may be arranged suitable for the electric grid. An example of this kind of equipment is a frequency converter, which receives voltage of variable frequency and amplitude and correspondingly supplies voltage at a desired frequency and amplitude to be further transferred to the grid.
The modular structure in connection with a frequency converter denotes that the frequency converter consists of a plurality of partial converters. It is more feasible to implement a high-power frequency converter by coupling a plurality of partial converters in parallel than by providing one large converter. Implementation of one converter having sufficiently high power is often technically extremely complicated, because voltage and current capacities of the required power semiconductors set limits to powers to be used.
The modular structure also enables redundancy of electric drive. The redundancy is an important feature in connection with wind power plants, in particular, as they are often located in sites where maintenance is difficult. The modular structure makes it possible that a fault in one part of the electric drive does not necessarily interrupt generation of electricity in the entire wind power plant.
Publication EP-A-1523088 discloses a modular structure in connection with electric drives, and in particular, a modular structure that enables mains converter parts being added to or removed from use depending on the circumstances. The disclosed structure makes it possible to optimize effects on the network by using standard components, i.e. such mains converters and inverters that are designed to operate with one another in all operating conditions. Typically the mains converter is designed to have a nominal power that is higher than that of an inverter part intended in connection thereof. The difference is due to the requirements set for the mains converter part as regards reactive power transmission and supply voltage variations. A disadvantage with the known solution is that power steps in the mains converter are wide, and consequently at low transmitted powers, in particular, an undesirable amount of current harmonics is transferred to the network.